The present invention relates to a harmonically damped oscillator circuit.
Oscillator circuits are used for clocking in analog as well as digital electronic circuits. In the context, a quartz element is typically used for frequency stabilization, the quartz element being capable of being connected in parallel to an oscillator amplifier in a so-called xcfx80-structure. The external quartz is necessary for precisely selecting the frequency, while the internal oscillator amplifier supplies the needed energy for maintaining the oscillation.
This circuit engineering involves large amplitudes of the quartz oscillation. Consequently, the non-linear large-signal response of the oscillator amplifier result in a high harmonic component of the quartz oscillation and/or a high distortion factor, which, for the known oscillators, is in the order of 5%. Such harmonic waves are noticeable in connection with the large amplitudes as parasitically conducted interference of sensitive, analog signals, in particular when the oscillator signal is directed by a pad structure and pin structure, which is large in comparison with other components. The high distortion component and large amplitude of such a known oscillator, therefore, result in a decrease in quality and yield.
An amplitude control system for attenuating the harmonic distortion of an oscillator that is achieved with the aid of operational amplifier circuits and multipliers is known from G. J. Fortier and I. M. Filanovsky, xe2x80x9cA linearized model of a twin-T RC oscillator employing an amplitude control system with multipliersxe2x80x9d, Int. J. Electronics, Vol. 61, No. 5, pgs. 617-625, 1986. This amplitude control system is designed for use in the audio range (frequencies in the KHz range) and no longer functions reliably at higher frequencies. Furthermore, the oscillator is produced using discrete components for which it is possible to precisely adjust the parameter values. However, such discrete circuits having individually calibrated components are complicated and expensive.
A low-distortion oscillator in the MHz range is described in A. Benjaminson, xe2x80x9cBridge Circuits enhance Crystal-Oscillator Stabilityxe2x80x9d, Microwaves and RF, Vol. 34, No. 11, pgs. 85-97, 1995. However, this design approach also requires a plurality of discrete elements, among other things, a so-called hot carrier diode, as part of the amplitude control system. This design is, therefore, too expensive for mass production of application-specific, integrated circuits (ASICs). Moreover, the amplitude of 800 mV used in this design is much too high.
The circuit known from B. Harvey, xe2x80x9cOscillators blend Low Noise and Stable Amplitudexe2x80x9d, Microwaves and RF, Vol. 33, No. 13, pgs. 125-129, 1994 also necessitates a plurality of discrete elements, in some instances, even a plurality of integrated circuits. The amplitude is also too high at about 500 mV.
E. A. Vittoz et al., xe2x80x9cHigh-Performance Crystal Oscillator Circuits: Theory and Applicationxe2x80x9d, IEEE Journal of Solid-State Circuits, Vol. 23, No. 3, June 1988, pgs. 774-783 describe an oscillator that can be integrated in a CMOS and is above all intended to enable frequency stability and low power consumption and proposes an amplitude control system for this purpose. However, this design uses low-voltage processes (1.1 V), and the transistors of the amplitude control system function in weak inversion. Due to the low currents, this control is too sensitive with respect to supply-voltage ripples as they occur in larger mixed-signal integrated circuits, which unite digital and analog circuits.
An amplitude control via a rectifier is proposed in U. Tietze, C. Schenk xe2x80x9cHalbleiterschaltungstechnikxe2x80x9d, 10th edition. However, in CMOS technology, the switching threshold of a rectifier can only be very roughly adjusted. Moreover, the voltage drop of a diode (about 500 mV) goes directly into the switching threshold and, as such, also into the amplitude. Thus, this circuit design for producing a harmonically damped oscillator circuit is too inexact for sensitive signal processing integrated circuits.
None of the known oscillator circuits can be produced in an integrated form and functions reliably and stably even in the case of large degrees of scatter in the individual component characteristics due to fluctuations in the process parameters. Large fluctuations in temperature from xe2x88x9240xc2x0 C. to +125xc2x0 C. and supply ripples of several megahertz also affect the function of the known oscillator circuits.
The harmonically damped oscillator circuit according to the present invention has a controllable oscillator amplifier for producing an oscillator output signal and an amplitude control circuit for controlling the amplitude of the oscillator output signal. The amplitude control circuit generates an amplitude control signal as a function of the determined amplitude of the oscillator output signal in such a manner that the oscillator amplifier functions in a preset operating range at a predefined operating point in which the oscillator amplifier functions linearly and, therefore, produces a low harmonic component, the oscillator amplifier being designed in such a manner that the preset operating range and the preset operating point are independent of the amplitude control signal Vcontrol. The amplitude can be variably set in a range of about 50-200 mV and is stable in the case of parameter fluctuations such as temperature fluctuations or interference. As a result, the amplitude can be reliably and stably controlled even using components that cannot be individually set or calibrated and whose electronic characteristics have a certain degree of scatter due to the manufacturing process.
The amplitude control signal can be fed directly to the oscillation amplifier itself or can set the resistance value of a feedback resistor connected in parallel to the oscillator amplifier.
The amplitude control circuit can have a comparator circuit and a low-pass filter, the comparator circuit producing a pulse width modulation signal by comparing the amplitude of the oscillator output signal to a predefined threshold value, the signal then being input into the low-pass filter to generate the control signal. In this context, the low-pass filter is preferably a higher order filter, in particular third order, the switching edges of the pulse width modulation signal being so strongly attenuated by the comparator circuit that the oscillator output signal is not affected by high frequency components of the control signal. The switching time of the comparator circuit should, therefore, be short with respect to the oscillator period.
According to a variant of the oscillator circuit according to the present invention, the comparator generates a pulse width modulation signal whose pulse duty ratio is:                     T        0            T        =                                        1            π                    ·          acos                ⁢                  xe2x80x83                ⁢                  S          A                ⁢                  xe2x80x83                ⁢        for        ⁢                  xe2x80x83                ⁢        A             greater than       S        ,      xe2x80x83    ⁢            and      ⁢                        xe2x80x83                ⁢                  xe2x80x83                    ⁢      0      ⁢              xe2x80x83            ⁢      for      ⁢              xe2x80x83            ⁢      A        ≤    S  
T being the period duration of the oscillator and T0 being the duration of the low-level state of the pulse width modulation signal within a period.
The pulse width modulation signal then has low-level pulse segments when amplitude A exceeds the predefined threshold value S, the portion of low-level segments of an entire period increasing up to xc2xd when the amplitude of the oscillator output signal increases to a value significantly greater than S. This pulse width modulation signal is then fed through the low-pass filter to generate the xe2x80x9csmoothxe2x80x9d amplitude control signal Vcontrol.
Preferably, the comparator circuit has a plurality of series-connected inverters, since very short switching times are to be achieved with CMOS inverters. A sufficient amplification can be achieved with the aid of a series-connected inverter cascade. By suitably dimensioning the first inverter, the effect of the comparator on the oscillator signal can be minimized (miller effect).
By suitably selecting threshold value S, amplitude A of the oscillator output signal can be suitably selected, an amplitude between 50 mV and 200 mV, in particular approximately 100 mV, being preferred. The dimensioning of the first inverter defines switching threshold S. The characteristic of xe2x80x9cmatchingxe2x80x9d integrated components causes S to remain almost constant even in response to parameter fluctuations (matching between transistors of the oscillator amplifier and transistors of the first compensator inverter).
The oscillator circuit according to the present invention can be advantageously produced as an integrated circuit or part of an application-specific integrated circuit (ASIC).